Re: [Qemu-devel] DIRTY_MEMORY_BLOCK_SIZE;

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefan Hajnoczi (stefanha@redhat.com) wrote:
> On Fri, Jun 30, 2017 at 12:26:10PM +0100, Dr. David Alan Gilbert wrote:
> > * ali saeedi (ali.saeedi56@gmail.com) wrote:
> > > Hello
> > > what does 'DIRTY_MEMORY_BLOCK_SIZE' mean?
> > > is it the number of words in a block? or number of pages in a block? or
> > > number of bytes in a block?
> > > thanks a lot
> > 
> > (cc'ing Stefan)
> > I think that DIRTY_MEMORY_BLOCK_SIZE is the number of TARGET_PAGEs
> > within one DIRTY_MEMORY_BLOCK
> > So with the common 4k target page that's 4k*256k*8=8GB/dirty memory
> > block - note these are just the size of structure sin qemu, it's still
> > got the ranularity ot mark individual target pages as dirty.
> 
> Right, the calculation is shown in the comment above the code:
> 
>  *   rcu_read_lock();
>  *
>  *   DirtyMemoryBlocks *blocks =
>  *       atomic_rcu_read(&ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]);
>  *
>  *   ram_addr_t idx = (addr >> TARGET_PAGE_BITS) / DIRTY_MEMORY_BLOCK_SIZE;
>  *   unsigned long *block = blocks.blocks[idx];
>  *   ...access block bitmap...
>  *
>  *   rcu_read_unlock();
> 
> Rather than focussing on DIRTY_MEMORY_BLOCK_SIZE, make sure you
> understand how DirtyMemoryBlocks works.  It is an array of bitmap
> pointers.
> 
> Instead of directly indexing into a single huge dirty memory bitmap,
> QEMU divides the dirty memory bitmap into fixed-sized chunks.  Each
> chunk covers DIRTY_MEMORY_BLOCK_SIZE pages.
> 
> The reason for this layer of indirection is so that the dirty memory
> bitmap can be accessed without taking a traditional lock (just RCU) and
> also supports memory hotplug.
> 
> Without indirection it would be difficult to grow the bitmap while other
> threads are writing to it.  Thanks to the indirection, it's possible to
> allocate new chunks and continue using the old chunks when growth
> occurs.

I guess this works like the old non-chunk version, in that there's no
direct correspondence between DirtyMemoryBlocks and RAMBlock's - i.e.
one RAMBlock might span two DirtyMemoryBlocks even if it's smaller
than DIRTY_MEMORY_BLOCK_SIZE.

Dave


--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK